1. Technical Field of the Invention
This invention relates generally to electromagnetic transducers such as audio speakers, and more specifically to a transducer having a geometry allowing greater linear travel and magnetic braking.
2. Background Art
Speakers are shown in cross-section in this document. Because speakers are generally cylindrically or rotationally symmetrical about an axis line or center line, only one side of any given speaker is shown, but the skilled reader will readily appreciate the three-dimensional structure which is thus represented. The reader will appreciate, however, that the invention is not limited to such axially symmetric implementations.
FIG. 1 illustrates a conventional audio speaker 10 such as is known in the prior art, shown as symmetrical about a center line CL. The speaker includes a magnetically conductive pole plate 12 which includes a pole 14 which may be either coupled to or integral with the base or back plate 16 of the pole plate, as shown. The pole may include an axial hole 18 for permitting airflow to cool the motor structure and depressurize the diaphragm assembly. A ring-shaped permanent magnet 20 surrounds the pole, with a cavity 22 between them. A magnetically conductive top plate 24 surrounds the pole, with a magnetic air gap 26 between them. Typically, the magnetic air gap will be smaller than the cavity. The pole plate, magnet, and top plate may collectively be termed a magnet assembly or a motor structure. The heavy black arrows denote exemplary directions of flux flow, throughout this document; the skilled reader will readily appreciate that the magnets may be reversed, and the flux will flow the opposite direction, and the transducer will operate correctly, especially when provided with an inverse phase electrical input signal.
An electrically conductive voice coil 28 is rigidly attached to a cylindrical bobbin or voice coil former 30. The voice coil is suspended within the magnetic air gap to provide mechanical force to a diaphragm 32 which is coupled to the bobbin. When an alternating current is passed through the voice coil, the voice coil moves up and down in the air gap along the axis of the speaker, causing the diaphragm to generate sound waves.
A frame 34 is coupled to the magnet assembly. There are two suspension components. A damper or spider 36 is coupled to the bobbin and the frame, and a surround 38 is coupled to the diaphragm and the frame. These two suspension components serve to keep the bobbin and diaphragm centered and aligned with respect to the pole, while allowing axial movement. A dust cap 40 seals the assembly and protects against infiltration of dust particles and other stray materials which might contaminate the magnetic air gap and thereby interfere with the operation or quality of the speaker.
When, as shown, the voice coil is taller (along the axis) than the magnetic air gap, the speaker is said to have an “overhung” geometry. If, on the other hand, the voice coil were shorter than the magnetic air gap, the speaker would be “underhung”. If they are equal, the speaker is “equalhung”.
If the voice coil moves so far that there exists a different number of voice coil turns within the air gap (i.e. an overhung voice coil has moved so far that one end of it has entered the air gap, or an underhung voice coil has moved so far that one end of it has left the air gap), the speaker begins to exhibit nonlinear characteristics, and the sound quality is distorted or changed. This is especially problematic when playing low frequency sounds at high volume, which require maximum voice coil travel.
The common approach to solving this problem has been to use highly overhung or highly underhung geometries to achieve a large amount of linear voice coil travel. These approaches have inherent limitations, however. The highly overhung motor requires increasingly longer coils, which in turn increases the total moving mass of the diaphragm assembly. At some point, this ever-increasing mass becomes so great that the inherent mechanical design limits are reached, which prevents any further controllable increase in travel. At the same time, increasing the voice coil mass with no resultant increase in utilized magnetic flux will reduce the overall efficiency of the transducer. Efficiency is proportional to BL squared, and inversely proportional to mass squared. In the highly underhung geometry, other practical limits are reached because of the relative increase in magnet area required to maintain a constant B across the magnetic gap height in order to achieve higher linear travel without sacrificing efficiency. Unfortunately, this increase in available magnetic flux does not result in an increase in BL, and therefore the transducer's efficiency also does not increase.
FIGS. 2–4 illustrate the prior art such as that taught in U.S. Pat. No. 4,783,824 to Kobayashi, which uses a “push-pull” geometry in which the magnetic flux over the top magnetic air gap travels in the opposite direction as the flux over the bottom magnetic air gap. This requires that the two voice coils be wound in opposite directions as indicated by the conventional “point” and “tail” markings, or that the voice coils be driven with opposite phase input signals, either of which complicates the manufacturing process.
The Kobayashi speaker 42 includes a center pole 44 which is mechanically coupled to an external shield 46 by a high magnetic reluctance spacer 48. A drive magnet 50 provides magnetic flux to an upper drive plate 52 and a lower drive plate 54, in opposite directions. A bucking magnet 56 is coupled between the shield and the lower drive plate, and helps cancel any stray flux that would otherwise escape the motor. An upper voice coil 58 rides in an upper drive magnetic air gap 60 between the center pole and the upper drive plate, and a lower voice coil 62 of opposite winding or phase rides in a lower drive magnetic air gap 64 between the center pole and the lower drive plate. The voice coils are wound around a bobbin 66 which is mechanically coupled to the diaphragm and other parts (not shown) of the diaphragm assembly.
Xmax may be interpreted to mean the one-way travel (typically expressed in terms of extension rather than retraction) over which the transducer exhibits a substantially constant, maximized BL. When the voice coil assembly moves too far in either direction, the active BL will begin to roll off, e.g. when the trailing end of an overhung voice coil begins to enter the magnetic air gap and less than the full gap is actively used. Xmax may alternatively be interpreted to include some greater distance than this, and is often interpreted to mean that distance over which the transducer exhibits no more than 10% distortion, or no more than 30% rolloff in active BL, or the like. In many instances in this document, we mean Xmax to mean the former (the maximum flat table-top portion of the BL curve), but the invention should not be interpreted to be narrowly limited to this case.
The maximum one-way linear travel (Xmax) can be observed as follows. The drive plates each have a thickness or height Hmg, while the voice coils each have a height Hvc. With the voice coil assembly at rest (not being electrically powered), the top of the upper voice coil is at a resting position Trest, as shown in FIG. 2. FIG. 3 illustrates that as the voice coil is driven outward to its maximum linear extension, where the bottom of each voice coil is just entering its magnetic air gap, the top of the upper voice coil has extended to a maximum extension point Tout. The distance from Trest to Tout is Xmax. FIG. 4 illustrates that as the voice coil is driven inward to its maximum linear retraction, where the top of each voice coil is just entering its magnetic air gap, the top of the upper voice coil has retracted to a maximum retraction point Tin. The distance from Trest to Tin is also Xmax, and the distance from Tout to Tin is Xtotal.
The maximum one-way linear travel for this speaker is       X    ⁢                  ⁢    max    =            Hvc      2        -          Hmg      2      or, in other words,Xtotal=Hvc−Hmg
Throughout the linear operation range of prior art push-pull overhung speakers, it is always the case that 100% of the total height of available magnetic air gap is active and less than 100% of the total height of available voice coil is active. Throughout the linear operation range of prior art push-pull underhung speakers, it is always the case that 100% of the total height of available voice coil is active and less than 100% of the total height of available magnetic air gap is active.
Speakers may generally be classified as having an external magnet geometry (in which ring magnets surround a pole plate) or an internal magnet geometry (in which a cup contains magnets). Pole plates and cups may collectively be termed magnetic return path members or yokes, as they serve as the return path for magnetic flux which has crossed over the magnetic air gap.
Materials may be classified as either magnetic materials or non-magnetic materials. Non-magnetic materials may also be termed non magnetically conductive materials; aluminum and chalk are examples of non-magnetic materials. Magnetic materials are classified as hard magnetic materials and soft magnetic materials. Hard magnetic materials are also called permanent magnets, and generate magnetic flux fields without outside causation. Soft magnetic materials are those which, although not permanent magnets, will themselves become magnetized and conduct flux in response to their being placed in a magnetic field. Soft magnetic materials include the ferrous metals such as steel and iron.